How do glaciers erode the land?
When gravity causes ice to move down the side of a mountain, there are two ways in which the rock below is eroded. These are plucking and abrasion. Freeze-thaw weathering is a process that also affects glacial environments.
Plucking is when meltwater from a glacier freezes around lumps of cracked and broken rock. When the ice moves downhill, rock is plucked from the back wall. Plucking is particularly effective when the rock contains joints (cracks) that water can seep into. Meltwater is found under a glacier due to the weight of the ice. The tremendous pressure causes ice at the base of the glacier to melt. This is known as pressure melting. Also, meltwater flows from the glacier’s surface to its base through large cracks, known as crevasses, in the ice.
Abrasion is when rock is frozen to the base, and the back of the glacier scrapes the bedrock. This acts like sandpaper and erodes the bedrock. Very large boulders can do enormous damage. Large scars created in the bedrock by this process are called striations.
A large amount of plucking increases the abrasion rate as more rock is embedded in the base of the glacier.
At the top end of the glacier, the ice doesn’t move in a straight line — it moves in a circular motion called rotational slip. This can erode hollows in the landscape and deepen them into bowl shapes called corries.
Weathering is the breakdown of rock in situ (in the place where it is). Freeze-thaw is when meltwater or rain gets into cracks in the bedrock, usually the back wall. At night the water freezes, expands and causes the crack to get larger. Eventually, the rock will break away.
In glaciated areas, freeze-thaw weathering (or frost shattering) occurs on rock surfaces above the surface of the ice and at its margins.
The evidence for freeze-thaw weathering is seen in landscape features called scree slopes and blockfields. These are piles of rock that cover large upland areas in the UK. Some scree slopes and blockfields date from the last ice age; others were more recently formed.
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